Lymphocyte function-associated antigen-1 (LFA-1) (alphaLbeta2, CD11a/CD18), the target receptor of this invention, is a member of the integrin superfamily with 24 members known todate. Integrins are heterodimeric cell-surface receptors composed of one alpha subunit and one beta subunit each. Functionally, integrins are cellular adhesion molecules mediating a wide range of cell-cell, cell-extracellular matrix and cell-pathogen interactions. Within the integrin superfamily, LFA-1 belongs to the beta 2 integrin subfamily, which is defined by a common beta 2 chain and unique alpha chains. The four members of the beta2 integrin subfamily are alphaLbeta2 (LFA-1, CD11a/CD18), alphaMbeta2 (Mac-1, CD11b/CD18), alphaXbeta2 (gp 150, CD11c/CD18) and alphaDbeta2 (CD11d/CD18) [Tan 2012]. A condition termed “Leukocyte Adhesion Deficiency-I” (LAD-I) has been identified in patients having a deficiency in beta2 (CD18) integrins. LAD-I is characterized by serious bacterial and fungal infections.
The expression of LFA-1 is restricted to leukocytes including T-cells, B-cells, neutrophils, monocytes, macrophages, dendritic cells, mast cells, eosinophils, and NK cells. The level of expression varies with cell type and differentiation state. LFA-1 is overexpressed in certain lymphomas and leukemias [Poria 2006, Chittasupho 2010]. The major ligands of LFA-1 identified to date belong to the immunoglobulin (Ig) superfamily. They are the intercellular adhesion molecules ICAM-1, ICAM-2, ICAM-3, ICAM-5 and the junctional adhesion molecule JAM-A (previously JAM-1). These ligands are expressed on various cell types including endothelial cells lining the vessel wall, epithelial and tissue resident cells (e.g. keratinocytes, dendritic cells) and leukocytes [Tan 2012].
LFA-1 plays a central role in the innate and adaptive immune response. Firstly, as a cellular adhesion molecule LFA-1 mediates the firm adhesion of leukocytes to inflamed vessel walls and their extravasation into inflamed tissues. Secondly, LFA-1 is crucial for the activation of immune cells. In this context, LFA-1 is well-characterized as a co-stimulatory receptor which is essential for the formation of the immunological synapse and controls T cell activation and proliferation. The central roles of LFA-1 in the immune response require tight control of LFA-1 activation. Normally, LFA-1 resides on the cell surface in an inactive state. Upon intracellular signaling (so-called “inside-out” signaling) LFA-1 is converted from an inactive to an active, ligand-binding state. This conversion is associated with major conformational changes within the receptor. Upon ligand binding, LFA-1 conveys signals back into the cells (so-called “outside-in” signaling), triggering subsequent steps which depend on the cell type [Hogg 2011].
Given its broad distribution on immunocompetent cells, LFA-1 plays a central role in immune-mediated and inflammatory diseases. This has been established extensively in experimental disease in animals, mostly using knock-out mice and anti-LFA-1 antibodies. Anti-LFA-1 therapy led to prolonged graft survival in various models of allograft transplantation (including cardiac, islet and cornea transplantation). Moreover, in several transplantation models, tolerance could be induced with both anti-LFA-1 therapy used alone or in combination with other modalities [Nicolls 2006, Arefanian 2010]. In other experimental disease models, for example, uveitis, arthritis, multiple sclerosis, diabetes mellitus, asthma and lupus-like disease animals genetically deficient for LFA-1 or treated with anti-LFA-1 agents were found to be protected against disease [Giblin 2006, Ke 2007, Glawe 2009, Lee 2008, Suchard 2010]. LFA-1 has also been identified as a therapeutic target for infectious diseases, including HIV infection [Kapp 2013].
Beyond, LFA-1 has been described as target receptor for drug delivery or for the delivery of marker molecules such as imaging agents (diagnostic usage) to lymphoma and leukemia cells [Chittasupho 2010, Poria 2006].
Further, LFA-1 plays decisive roles in the differentiation of lymphocyte populations [Verma 2012] and may be used as a target allowing the selection or expansion of distinct lymphocyte populations in vitro, ex vivo and in vivo. Increases in regulatory lymphocyte populations have been observed in patients treated with anti-LFA-1 antibodies [Faia 2011, Posselt 2010].
The clinical experience with LFA-1 targeting therapies largely derives from the use of monoclonal anti-LFA-1 antibodies, i.e. odulimumab used for the treatment of graft-versus-host disease [Jabado 1996] and efalizumab for plaque psoriasis [Lebwohl 2003], primarily, with small pilot studies in other autoimmune indications [e.g. Usmani 2007, Navarini 2010, Faia 2011]. Efalizumab was withdrawn from markets because of the occurrence of four cases of a rare, often fatal viral infection of the brain (progressive multifocal leukoencepahlopathy) [Seminara 2010].
Several small molecule inhibitors have been described in patent applications or scientific literature which affect the interaction of LFA-1 with their ligands [Giblin 2006]. To date, these compounds can be grouped into two major classes, based on how they bind to LFA-1 and how they influence LFA-1 conformation. One class of inhibitors, termed alpha I allosteric inhibitors, binds to the ligand binding domain (termed I domain) on the LFA-1 alpha chain, however at a site distal to the ligand binding site (termed alpha L I allosteric site). These inhibitors stabilize LFA-1 in its bent inactive state, preventing the switchblade-like opening of LFA-1 into its extended active state, and the exposure of activation epitopes [Giblin 2006]. Furthermore, the binding of alpha I allosteric inhibitors to LFA-1 can be elegantly detected by the loss of the mAb R7.1 epitope [Weitz-Schmidt 2004]. Major chemical classes of alpha I allosteric inhibitors which have been described so far include hydantoin derivatives, statin (or “mevinolin”)-based derivatives, substituted diazepanes and arylthio cinnamide analogues [for review see Giblin 2006 and Kapp 2013]. None of these compounds is reported to be in clinical development, currently.
Another group of inhibitors, termed alpha/beta I allosteric inhibitors, are ligand mimetics, i.e. they are derived from amino acids of the LFA-1 binding region of ICAM [Giblin 2010, Kapp 2013]. Unexpectedly, these ligand mimetics do not bind to the ligand binding site of LFA-1 located on the alpha chain. Instead, they act via the LFA-1 beta chain by competing with the interaction of an internal ligand. As a result the binding domain of LFA-1 remains in a low affinity state whereas the rest of LFA-1 adapts an extended, active conformation, exposing activation epitopes [Giblin 2010]. Cells treated with these inhibitors exhibit paradoxical activities, i.e. they induce “rolling adhesion”, in contrast to cells treated with alpha L I allosteric inhibitors [Salas 2004]. Paradoxical agonism has also been observed with ligand mimetics targeting other integrin family members including alpha IIb/beta3 [Ahrens 2008]. Currently, one ligand mimetic LFA-1 inhibitor is in clinical development for the treatment of dry eye disease [Sheppard 2014, Kapp 2013].
Taken together, it is appreciated that LFA-1 is a receptor involved in inflammatory, immune-mediated and infectious diseases and is overexpressed in certain malignant diseases. These diseases are often severe, chronic disorders often requiring life-long therapy. From a clinical perspective, there remains a high need for effective therapies either preventing the conditions or controlling the activity of disease and providing long-term benefit/risk profiles superior to currently available therapies. It is further appreciated that novel LFA-1 inhibitors with improved pharmacologic profiles and devoid of side effects observed with earlier LFA-1 inhibitors will constitute a therapeutic advance.
In conclusion, based on the status of prior art, there remains a need for novel, improved LFA-1 inhibitors of chemical scaffolds different to scaffolds described before. The availability of such inhibitors would provide additional therapeutic and diagnostic options across the spectrum of diseases in which LFA-1 is involved or can be employed as a target for drug delivery, given that different chemical structures will be associated with different pharmacologic profiles. Furthermore, it is appreciated that the function of LFA-1 can be modulated in different ways and that unwanted effects/side-effects observed with certain classes of existing LFA-1 inhibitors, such as anti-LFA-1 monoclonal antibodies, may not be observed with novel types of LFA-1 inhibitors.